With higher integration of semiconductor devices, the miniaturization of their device pattern progresses. For example, 1-gigabit dynamic random access memory (DRAM) has been used in practical applications. Such large capacity DRAM employs elements reduced in dimension and hence in surface area. Since DRAM uses the amount of charge stored on its memory cell capacitors as stored information, these capacitors must have a capacitance greater than a certain value. Therefore, memory cell capacitors currently have a very high aspect ratio. Conventionally, CVD (Chemical Vapor Deposition) is used to form capacitive dielectric films for capacitors. However, it is difficult by CVD to uniformly form a dielectric film in a high aspect ratio groove with a high coverage. In order to retain its stored data, a memory cell capacitor must have high capacitance and a low leakage current, which requires the formation of the thinnest possible uniform film.
In recent years, in order to solve this problem, ALD (Atomic Layer Deposition) has been used, which repeatedly deposits a thickness of a film material on the order of an atomic layer to form a film having a desired thickness.
A process, which repeatedly depositing a thickness of a film material on the order of a molecular layer to form a film having a desired thickness, is referred to as MLD (molecular layer deposition) to be distinguished from ALD in some cases. However, in this specification, the both techniques are commonly referred to as ALD, since they are based on the same principle. In a case where a hafnium oxide (hereinafter referred to as an “HfO”) film is formed on a semiconductor substrate by ALD, a gas supply cycle consisting of the supply of tetrakis(ethylmethylamino)hafnium (hereinafter abbreviated as “TEMAH”), which is an organic metal material, and the supply of ozone (O3) is repeated a plurality of times while maintaining the semiconductor substrate at a predetermined temperature. The reaction vessel is evacuated and purged by inert gas after the supply of one gas and before supply of the other to ensure that the gases react only on the semiconductor substrate. An HfO film having a thickness on the order of an atomic layer is formed on the semiconductor substrate during each cycle. The above gas supply cycle is repeated a predetermined number of times determined depending on desired thickness, so that an HfO film can be formed with excellent film-thickness reproducibility.
ALD is advantageous in its excellent film-thickness reproducibility, but is disadvantageous in its long film-forming time. For example, in forming an HfO film, one cycle deposits a film of approximately 0.1 nm. Thus, 50 cycles are required to form a film of thickness of 5 nm. If each cycle takes 1 minute, the total film forming time will be approximately 50 minutes. Therefore, the use of a batch-type system is preferable to a single substrate processing system in terms of productivity.
FIG. 5 shows the configuration of a conventional batch-type ALD system for forming an HfO film; and FIG. 6 shows a piping system associated with this system. Referring to FIG. 5, a vacuum exhaust port 1103 is provided at the top of a reaction tube 1102 defining a reaction chamber 1101. The vacuum exhaust port 1103 is connected to a vacuum pump 1105 through an evacuation pipe provided with a pressure adjusting valve 1104. A boat 1108 having a plurality of semiconductor substrates 1107 mounted therein is supported on a boat loader 1106 and loaded in the reaction chamber 1101. A heater 1109 is provided around the reaction tube 1102 to heat the semiconductor substrates.
Liquid TEMAH is supplied from a TEMAH supply source 1110 through a liquid flow rate adjuster 1111 to a vaporizer 1112, in which the liquid TEMAH is vaporized and a TEMAH gas, as a source gas, is then delivered into the reaction chamber 1101 through a TEMAH nozzle 1113. Nitrogen gas (N2) is supplied from a nitrogen supply source 1114 to the vaporizer 1112 through a flow rate adjuster 1115 to aid vaporization of the liquid TEMAH. Oxygen gas (O2) is supplied from an oxygen supply source (not shown) through a flow rate adjuster (not shown) to an ozone generator (not shown) in which the oxygen gas is converted into ozone which is an oxidizer. The ozone is then delivered into the reaction chamber 1101 through an ozone nozzle 1113. Further, nitrogen gas used as a purge gas is supplied from the nitrogen gas supply source 1114 to the reaction chamber 1101 through a flow rate adjuster 1116 and a nitrogen gas nozzle 1113. There are plural nozzles 1113 each dedicated to a different gas, although only one nozzle 1113 is shown in FIG. 5 for simplicity.
When HfO films are formed on the semiconductor substrates using the system having the configuration shown in FIG. 5, there may be a difference in thickness or quality of the films between the semiconductor substrates mounted in the lower portion and the upper portion of the boat 1108. The reason is that the semiconductor substrates disposed in the upper portion of the reaction chamber 1101 cannot receive sufficient amounts of TEMAH gas and ozone, since the TEMAH gas and the ozone delivered from L-shaped nozzles 1113, which are provided in the lowest portion of the reaction chamber 1101, are mostly consumed by the reaction occurring in the lower portion of the reaction chamber 1101. Unlike CVD, in ALD, the supply of excessive amount of source gas does not result in formation of an unnecessarily large film thickness. Therefore, excessive TEMAH gas may be supplied into the reaction chamber 1101 to cause a sufficient amount of TEMAH gas to reach the upper portion of the reaction chamber 1101. On the other hand, ozone has a short life under elevated temperature conditions. Therefore, the supplied ozone progressively disappears as it flows from the lower portion to the upper portion of the reaction chamber 1101 and, as a result, the upper portion of the reaction chamber 1101 is more likely to lack ozone. To solve this problem, it may be conceived that an excessive amount of ozone may be supplied to the reaction chamber 1101. However, the supply of an excessive amount of ozone causes oxidation damage to the components in the lower portion of the reaction chamber 1101, which is not desirable. Furthermore, ozone is consumed by this oxidation reaction.
In order to solve these problems, a distributing nozzle(s) 1117 such as shown in FIG. 7 may be used. The distributing nozzle 1117 extends from the bottom to the top of the reaction chamber 1101 and includes nozzle holes 1118 each corresponding to respective semiconductor substrates mounted in the boat 1118. This arrangement allows processing gas, especially ozone, to be uniformly supplied to the semiconductor substrates. It should be noted that the TEMAH gas may be supplied through an L-shaped nozzle shown in FIG. 5 or the distributing nozzle shown in FIG. 7.
Incidentally, particle reduction is a critical issue in semiconductor manufacturing. When a film is formed on the semiconductor substrates by CVD, ALD, or other chemical deposition process, deposition of unnecessary films unavoidably occurs on the inner wall of the reaction tube and on the various components that are exposed to the atmosphere within the reaction vessel. Peeling-off of the unnecessary films is a major cause of the generation of particles, as is well known to those of ordinary skill in the art. Peeling-off of the unnecessary films tends to occur when the unnecessary films have a large thickness or when the inside of the reaction vessel is exposed to the ambient atmosphere. For example, generation of a large quantity of particles was found after the inside of reaction vessel was exposed to the ambient atmosphere for maintenance or repair after performing deposition of HfO films for many times. Analysis of these particles by EDX (energy dispersive X-ray spectroscopy) revealed that they were formed of hafnium oxide. It is thought that the above generation of particles resulted from the fact that HfO films formed on the inner wall of the reaction tube absorbed moisture and thereby peeled off when the inside of the reaction tube was exposed to the ambient atmosphere for maintenance. It was not possible to visually recognize the HfO particles since their sizes were very small (mostly 10 microns or less).
One of the possible countermeasures against the generated particles is cycle purging. A trial was conducted to reducing particles by using the cycle purging. The purging was performed by repeating a cycle consisting of an ozone flowing step, an evacuating step, and a nitrogen gas flowing step 50 times (spending approximately 3 hours), as shown in FIG. 9. In FIG. 9, the horizontal axis is graduated in 15 sec increments. The ozone concentration in the ozone flowing step was 200 g/Nm3; the oxygen flow rate before the oxygen-to-ozone conversion (corresponding to the ozone flow rate) was 10 SLM; the pressure in the reaction vessel in the evacuating step was approximately 5 Pa; and the nitrogen gas flow rate in the nitrogen gas flowing step was 10 SLM. Dummy semiconductor substrates were placed in the boat and heated to 300° C. The number and distribution of particles on the dummy semiconductor substrates were observed after completion of every predetermined number of cycles. Various particle distribution patterns were found: locally concentrated patterns sparsely distributed patterns, etc. FIGS. 10A and 10B show two examples of the observed particle distribution patterns. FIG. 11 shows change in the number of particles. The number of particles was not stably reduced even after more than 200 purge cycles (spending approximately 12 hours). Under such conditions, a deposition process can not be performed.
Another possible countermeasure against the generated particles is cleaning. However, there is no established method for removing an HfO film by in-situ dry cleaning. Wet cleaning, on the other hand, requires disassembly of the system, resulting in significant downtime. Furthermore, wet cleaning should not be frequently performed, since it shortens the life of the quartz components. Replacement of components results in shorter downtime. However, it is not practical since quartz components are expensive.